The performance of radio receivers may be reduced by the multipath reception phenomenon. This occurs when the transmitted signal reaches the receiver antenna by a direct path as well as indirect paths through reflections from objects. Depending on the delay between the direct and the reflected signals, they may add destructively and thereby interrupt normal operation of the receiver. For broadcast systems, this phenomenon is often observed in FM receivers, where the wavelength of the radio signals and the distance to the surroundings (buildings, mountains, etc.) make this undesired multipath effect very noticeable to the user.
In stationary situations, the position of the radio can be changed by just a few centimeters to ensure the antenna is at a location where the signals add constructively and the multipath distortion is avoided. In car radios, however, the antenna of the radio receiver will be subjected to continuously changing conditions and therefore multipath reception remains a major limitation in the quality of the received audio. Digital Broadcasting, making use of multi-carrier orthogonal frequency-division multiplexing (OFDM) signals spread over a relatively wide frequency range, mitigates this shortcoming of analog FM radio. But due to slow worldwide penetration of digital radio standards, FM radio reception remains widely used by car radio users and manufacturers.
Therefore, in high-end applications, car manufacturers provide at least one additional antenna such that the two (or more) antennae will be separated by a distance that is about a quarter of the wavelength of the signals of interest. In this way, signals received by the two antennae are “uncorrelated” with respect to reflections—when one antenna is in a “signal dip” due to multipath interference, the other antenna should receive a strong signal. More advanced radio receivers apply phase diversity algorithms that make use of these uncorrelated signals at multiple antennae, adjusting signal delays in the receiver system such that signal addition is constructive before demodulation. This eliminates multipath distortion at the audio output. This may be achieved by providing separate tuners (radio front-ends) for each antenna and combining the signals digitally by a phase diversity algorithm prior to demodulation.
To achieve constructive addition of signals, all signal processing steps in the separated paths, prior to phase diversity operation, make use of clock and timing references derived from the same crystal reference. This is to say that, for example, radio frequency (RF) mixers in each antenna path converting signals to baseband are driven by Local Oscillator (LO) signals from the same phase locked loop (PLL) and/or reference crystal oscillator to the PLL(s). Small, but constant, phase offsets can be corrected for by most phase diversity algorithms. But even a small frequency offset, due to physically different crystals, translates to continuously changing phase that cannot be distinguished from differences in time of arrival at the antennae by phase diversity algorithms. This constraint limits the practical distance between the 2 (or more) receivers configured as phase diversity system.